The present invention relates to a diene rubber composition and a tire using the composition in the tread and more particularly, to a rubber composition which contains, as a main rubber component, an elastomeric butadiene homopolymer or elastomeric copolymer of butadiene and an aromatic vinyl compound prepared by a solution polymerization process using a coupling agent and which has a high wet skid resistance, a low rolling resistance and an excellent processability, as well as high tear strength and tackiness which are important in processing.
In recent years, the reduction of running fuel cost of automobiles is in strong demand from the viewpoint of energy saving. Especially, it is of urgent necessity for the tire manufacturing industry to decrease the rolling resistance of tires, because the rolling resistance has an important effect on fuel saving.
Among the constituent elements of a tire, a tread rubber accounts for the largest proportion of the travelling resistance dependent on rolling of the tire, and it is known that the tread rubber accounts for about 40% of the rolling resistance of the tire at large. The rolling resistance of a tire results from energy loss, accompanying repeated deformation of a running tire. Thus, the reduction of the rolling resistance is no more than reducing such an energy loss. Particularly, considering it with respect to the motion of a tread rubber, it is necessary to reduce energy loss due to a bending and shearing motion and a compressive motion. From the viewpoint of the dynamic viscoelastic characteristic of a rubber, this means reduction of the loss modulus (E") and the loss compliance [E"/(E*).sup.2 wherein E* is complex modulus]. Further, these motions are repeated motions dependent on rotation of a tire, and the frequency thereof is from 10 to 10.sup.2 Hz. Accordingly, it is necessary for the reduction of the rolling resistance of tires to decrease the loss modulus and loss compliance of a tread rubber in 10 to 10.sup.2 Hz.
On the other hand, considering from the safety side in running of a car, the wet grip characteristic is an important characteristic required for tires. It is a characteristic concerning travelling performances such as steering stability and breaking performance on a wet road, such as a road at the time of raining, and is a frictional resistance which is caused by contact of the treat surface of a tire with the road. In other words, since a tire shows a recovery from deformation with a time lag to a stress received from the road surface, a torque of a reverse direction to the running direction is generated. The resistance due to this torque is the frictional resistance, and is related to the loss coefficient (tan .delta.) from the viewpoint of the viscoelastic characteristic. Further, the frequency of the stress caused by the road is a very high frequency dependent on the unevenness of road, and according to the literature, the frequency is from 10.sup.4 to 10.sup.7 Hz. Therefore, in order to improve the wet grip characteristic of tires, it is necessary to raise the loss coefficient of a tread rubber in 10.sup.4 to 10.sup.7 Hz.
In a tread rubber compound generally and widely used heretofore, natural rubber, polyisoprene rubber, high-cis-1,4-polybutadiene rubber and styrene-butadiene rubber are mainly used as rubber components. The former three are superior in rolling resistance, but are poor in wet grip characteristic. On the other hand, a styrene-butadiene rubber is superior in wet grip characteristic, but is poor in rolling resistance. Therefore, a tread rubber composition which is excellent in both rolling resistance and the wet grip characteristic has not been obtained. A tread rubber composition has been used at the cost of either one property, or with keeping balance between the both properties by blending the above-mentioned elastomers. However, the present state is considered far from sufficient for the level demanded in recent years with regard to keeping a high wet grip characteristic and keeping a low rolling resistance at the same time.
The present inventors made a study of a polymer having excellent processabilities such as kneading processability, roll processability and extrusion processability required in tire manufacturing and a process for the preparation thereof as well as a fundamental study of the repugnant rolling and wet grip characteristics, and they found, as described in Japanese Patent Application No. 53387/1982, that a polymer having a particular structure has a high wet grip characteristic and an excellent rolling resistance characteristic together with an excellent processability. The proposed rubber compositions have excellent processability. The proposed rubber compositions have excellent features in having a high wet grip characteristic, a low rolling resistance and an excellent processability and in satisfying the level of tear strength and tackiness required in molding. The present inventors have continued the study in order to further improve these characteristics and have now reached the present invention.
It is an object of the present invention to provide a rubber composition suitable for use in the tread portion of tires.
A further object of the invention is to provide a rubber composition having improved wet grip characteristic, rolling resistance, processability, tear strength and tackiness.
These and other objects of the present invention will become apparent from the description hereinafter.